1. Field of the Invention
The present invention relates to a Magnetic Resonance Imaging (MRI) apparatus, and particularly relates to an MRI apparatus that suppresses a Magnetic Resonance (MR) signal generated from fat tissue by applying a fat-signal suppression method.
2. Description of the Related Art
A Magnetic Resonance Imaging (MRI) method is an imaging method of creating image data based on Magnetic Resonance (MR) signals generated along with excitation produced by exciting a nuclear spin in a substance placed in a static magnetic field with a high frequency pulse (Radio Frequency (RF) pulse) that has a Larmor frequency.
An MRI apparatus is a diagnostic imaging apparatus that creates image data based on MR signals detected from the inside of a living body, and can obtain various diagnostic information, such as biochemical information and functional diagnosis information as well as anatomical diagnosis information, so that the MRI apparatus is indispensable in a field of diagnostic imaging nowadays.
During an examination by using the MRI apparatus (an MRI scan), from living-body tissue including water tissue and fat tissue, MR signals generated from water tissue (i.e., MR signals arising from resonance of hydrogen nuclear spin in water tissue) and MR signals generated from fat tissue are  usually collected in a mixed manner. When creating image data from MR signals from water tissue, which are clinically effective, creation of image data of high quality may be difficult in some cases due to contamination by MR signals generated from fat tissue.
To solve this problem, there has been developed a method of suppressing MR signals generated from fat tissue by applying a fat-signal suppression method, such as an Inversion Recovery (IR) method or a so-called Chemical Shift Selective (CHESS) method. The IR method uses a difference between a longitudinal relaxation time of a hydrogen nuclear spin in water tissue and a longitudinal relaxation time of a hydrogen nuclear spin in fat tissue. According to the CHESS method, only magnetization of fat tissue is excited by using a difference between resonance frequencies of a hydrogen nuclear spin in different kinds of tissue, and a transverse magnetization component of the fat tissue is caused to disappear by a spoiler pulse (for example, JP-A H05-285116 (KOKAI)).
Moreover, to further suppress MR signals generated from fat tissue, there has been proposed a fat-signal suppression method by which a flip angle of a pre-saturation pulse for fat suppression (hereinafter, “fat-suppression pulse”), due to which MR signals generated from fat tissue substantially disappear, is set in accordance with scanning conditions (for example, U.S. Pat. No. 6,272,369).
As described above, by suppressing MR signals generated from fat tissue substantially completely by applying a fat-signal suppression method, such as the CHESS method, image data based on MR signals from water tissue can be created. However, in an actual clinical case, image data including water tissue and fat tissue in a mixed manner, precisely, image data based on MR signals from water tissue and MR signals from fat tissue of which level is suppressed to a desired level, is sometimes required.
For example, when imaging bone and ligament tissue by  performing an MRI scan onto a joint, MR signals collected from ligament tissue are originally feeble, and if a fat-signal suppression method is applied to the MRI scan, the MR signals collected from a bone that includes a number of fat components become extremely small. As a result, it becomes difficult to display bone and ligament tissue with a clear distinction.
Thus, an MRI scan to which a conventional fat-signal suppression method is applied has a problem that tissue having a low density of hydrogen nuclei, or tissue from which small MR signals are collected in relation to a relaxation time and a pulse sequence, and tissue including a large number of fat components cannot be separately displayed.